This invention relates to a robotic device and, more specifically, a passive actuator for maintaining a deburring tool against a workpiece.
Robots have been used for the application of metal removal tools against workpieces. Robots offer a flexibility and ability to provide consistent quality which, together with economic incentives, makes their use quite advantageous in various metal machining processes. However, certain metal removing processes, such as deburring and edge contouring, or the need for great accuracy in other processes require very sensitive control of the cutting force. Simple robot path programming is insufficient to provide the accuracy required for some metal removal processes. If the normal force between a deburring tool and workpiece exceeds a given level, rapid tool breakdown occurs and workpiece damage may result. On the other hand, if the normal force is too low, the tool will simply rub the workpiece without removing metal.
The need for very accurate control of the cutting force is especially significant in deburring and edge contouring of the quite expensive superalloy parts found in some aircraft engines. Such parts must be machined very accurately if they are to properly function under demanding conditions in aircraft engines.
Normal forces of a few ounces have been found to generate the desired 0.015 inch edge chamfers on superalloy aircraft engine components. Under certain conditions, it has been found that the normal cutting force applied to a workpiece by a particular deburring and edge contouring tool changes about one ounce per mil depth of cut. In other words, if the deburring force is to be held within a one ounce limit, one must program the path of the robot arm upon which the deburring and edge contouring tool is mounted to follow the workpiece surface to within one mil.
There are several problems which generally prevent one from programming a robot path to within one mil in order to machine the workpiece to the desired accuracy. In particular, programming robots to within one mil tolerances is not usually practical. Further, setting up workpieces to within one mil tolerances is quite costly. Even if these problems can be overcome, the robot motion is not a continuum. Instead, the robot motion is a series of incremental steps, which steps are often five mils or other increments larger than one mil. If the incremental steps are five mils, the open loop cutting force would vary an unacceptable five ounces.
Although feedback arrangements might be used in order to sense the normal force and control an actuator to maintain the force within prescribed limits, such feedback loop arrangements would require that the tool force sensor be connected to control a high precision actuator. Additionally, such an arrangement would have to have a fast response time and high positioning resolution. While such a device might compensate for a range of setup and robot path tolerance errors as well as the stepwise nature of the robot's motion, such an active (i.e., feedback) arrangement is relatively complex and expensive.